a) Field of the Invention
The present invention relates to a lens system which is to be used in video cameras, TV telephones, etc. and uses a radial type gradient index lens component.
b) Description of the Prior Art
In the recent years where there are prevailing video cameras and TV telephones using solid-state image pickup devices such as CCDs, there is a demand to reduce the number of lens components disposed in a lens system which is to be used in these optical apparatus for configuring the optical apparatuses more compact and lowering manufacturing costs thereof.
On the other hand, it is demanded to enhance optical performance of such a lens system as picture elements are arranged at higher densities on image pickup devices.
A lens system disclosed by Japanese Patent Kokai Publication No. Hei 7-181376, for example, is generally known as a lens system which has high optical performance and is usable in video cameras, TV telephones, etc. This conventional example is composed of ten to twelve lens elements so as to be a lens system having high optical performance.
Further, a lens system disclosed by Japanese Patent Kokai Publication No. Hei 5-134172 or Hei 6-230273 is known as another conventional example which uses a radial type gradient index lens component for reducing a number of lens components while maintaining high optical performance, and is composed of three lens components.
However, the conventional example mentioned above is configured for use in silver salt cameras and makes an amount of marginal rays insufficient when it is used in a lens system which uses a solid-state image pickup device since it has an exit pupil located extremely close to an image surface and does not allow nearly telecentric incidence of offaxial rays onto the image surface.
Further, a lens system disclosed by Japanese Patent Kokai Publication No. Hei 7-159697 is known as a conventional example of lens system which uses a gradient index lens component and is composed of three lens components. However, this conventional example is an objective lens system for microscopes using a radial type gradient index lens component and has a narrow field angle.
Furthermore, a lens system disclosed by Japanese Patent Kokai Publication No. Hei 4-97309 is known as a conventional example of lens system which has a relatively large field angle and is configured while taking telecentric incidence of offaxial rays into consideration. However, this conventional example has an extremely large difference in refractive index between an axial portion and a marginal portion of the gradient index lens component, whereby it is difficult to manufacture a material for the gradient index lens component.
Moreover, lens systems disclosed by Japanese Patent Kokai Publication No. Sho 52-29238 and No. Hei 5-107471 are known as conventional examples each of which is composed of two to five lens components. However, these conventional examples are objective lens systems for endoscopes which use radial type gradient index lens components and produce distortion in extremely large amounts.